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Critical Period Effects in Second Language Learning: The Influence of Maturational State on the Acquisition of English as a Second Language by J. S. Johnson and E. L. Newport

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  • COGNITIVE PSYCHOLOGY 21, 60% (1989)

    Critical Period Effects in Second Language Learning: The Influence of Maturational State on the Acquisition

    of English as a Second Language

    JACQUELINE %JOHNSON AND ELISSA L. NEWPORT

    University of Illinois

    Lenneberg (1967) hypothesized that language could be acquired only within a critical period, extending from early infancy until puberty. In its basic form, the critical period hypothesis need only have consequences for first language acqui- sition. Nevertheless, it is essential to our understanding of the nature of the hypothesized critical period to determine whether or not it extends as well to second language acquisition. If so, it should be the case that young children are better second language learners than adults and should consequently reach higher levels of final proficiency in the second language. This prediction was tested by comparing the English proficiency attained by 46 native Korean or Chinese speak- ers who had arrived in the United States between the ages of 3 and 39, and who had lived in the United States between 3 and 26 years by the time of testing. These subjects were tested on a wide variety of structures of English grammar, using a grammaticality judgment task. Both correlational and r-test analyses demon- strated a clear and strong advantage for earlier arrivals over the later arrivals. Test performance was linearly related to age of arrival up to puberty; after puberty, performance was low but highly variable and unrelated to age of arrival. This age effect was shown not to be an inadvertent result of differences in amount of experience with English, motivation, self-consciousness, or American identifica- tion. The effect also appeared on every grammatical structure tested, although the structures varied markedly in the degree to which they were well mastered by later learners. The results support the conclusion that a critical period for lan- guage acquisition extends its effects to second language acquisition. 8 1989 ACT- demic Press, Inc.

    In most behavioral domains, competence is expected to increase over development, whether gradually or in stages. However, in some domains, it has been suggested that competence does not monotonically increase with development, but rather reaches its peak during a critical period,

    This research was supported in part by NIH Grant NS16878 to E. Newport and T. Supalla, and by NIH Training Grant HD07205 to the University of Illinois. We are grateful to Geoff Coulter, Henry Gleitman, and all of the members of our research group for dis- cussion of the issues raised here, to Lloyd Humphreys for advice on statistical matters, to Marcia Linebarger for the loan of test materials, and to Carol Dweck, John Plavell, Dedre Gentner, Doug Medin, and two anonymous reviewers for helpful comments on earlier drafts of this paper. Reprint requests should be sent to Jacqueline S. Johnson, Department of Psychology, Cornell University, Uris Hall, Ithaca, NY 14850.

    In this paper we use the term crirical period broadly, for the general phenomenon of

    60 OOlO-0285/89 $7.50 Copyright 0 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

  • CRITICAL PERIOD 61

    which may be relatively early in life, and then declines when this period is over. For example, in the development of early visual abilities, the development of attachment, or-in the case considered here-the acqui- sition of language, it has been suggested that learners are best able to achieve the skill in question during a maturationally limited period, early in life. Elsewhere we have presented evidence that first language learning is indeed limited in this way (Newport & Supalla, 1987). The present paper focuses on the acquisition of a second language, asking whether this type of learning, undertaken only after a native language is already ac- quired, is nevertheless still maturationally constrained.

    We will begin by reviewing prior evidence on this hypothesis, for both first and second language learning, and will then present a new empirical study which we believe shows evidence for a maturational function in second language learning. Such evidence leaves open, however, whether the underlying maturational change occurs in a specific language faculty, or rather in more general cognitive abilities involved in language learning. We will conclude by considering the types of mechanisms which are consistent with our findings,

    Evidence for a Critical Period Effect in First Language Acquisition

    The critical period hypothesis, as advanced by Lenneberg (1967), holds that language acquisition must occur before the onset of puberty in order for language to develop fully. As will be detailed in the subsequent sec- tion, Lennebergs hypothesis concerned only first language acquisition; he left open the question of whether this critical period extended to sec- ond language acquisition, which would occur after a first language was already in place.

    Lennebergs argument contained two parts. First, he reviewed avail- able behavioral evidence suggesting that normal language learning oc- curred primarily or exclusively within childhood. At the time his book was written, no direct evidence for the hypothesis (from normal individ- uals who had been deprived of exposure to a first language for varying lengths of time in early life) was available. His review therefore included

    changes over maturation in the ability to learn (in the case under consideration in this paper, to learn language). We therefore include within this term maturational phenomena which other investigators have called sensitive, rather than critical, periods. By using the term in this broad fashion, we mean to avoid prejudging what the degree or quality of such matu- rational change may be (e.g., is it a sharp qualitative change vs. a gradual quantitative one?) and what the nature of the underlying maturational mechanism may be (e.g., is it a change in a special language faculty vs. a more general change in cognitive abilities?). These further questions will be addressed in part by the nature of our findings, and in part by future research.

  • 62 JOHNSON AND NEWPORT

    various types of indirect evidence, for example, differences in recovery from aphasia for children vs. adults, and differences in progress in lan- guage acquisition, before vs. after puberty, in the mentally retarded.

    Second, he proposed a mechanism which might be responsible for a maturational change in learning abilities. The proposed mechanism was fundamentally neurological in nature. He suggested that the brain, having reached its adult values by puberty, has lost the plasticity and reorgani- zational capacities necessary for acquiring language. Subsequent research has questioned whether all of the neurological events he cited occur at an appropriate time for them to serve as the basis for a critical period (Krashen, 1975). Nevertheless, the hypothesis that there is such a critical period for language learning has remained viable.

    Since Lennebergs writing, behavioral studies approximating a direct test of the critical period hypothesis for first language acquisition have become available. One such study is a well-known case of Genie, a girl who was deprived of language and social interaction until her discovery at the age of thirteen (Curtiss, 1977). Her lack of linguistic competence, particularly in syntax, after seven years of rehabilitation supports the critical period hypothesis. However, the abnormal conditions under which Genie was reared, including nutritional, cognitive, and social de- privation, have led some investigators to question whether her language difficulties have resulted only from lack of linguistic exposure during early life.

    More recently, Newport and Supalla (Newport, 1984; Newport & Su- palla, 1987) have studied language acquisition in the congenitally deaf, a population in which exposure to a first language may occur at varying ages while other aspects of social and cognitive development remain nor- mal. Their data come from congenitally deaf subjects for whom American Sign Language (ASL) is the first language. However, since 90% of the congenitally deaf have hearing (speaking) parents, only a few deaf indi- viduals are exposed to this language from birth. The majority of deaf people are exposed to ASL only when they enter residential school for the deaf and first associate with other deaf individuals; this can be as early as age four or as late as early adulthood.

    Newport and Supalla separated subjects by their age of exposure into three groups: native learners, who were exposed to ASL from birth by their deaf parents; early learners, who were first exposed to ASL between the ages of 4 and 6; and late learners, who were first exposed to ASL at age 12 or later. Wishing to test asymptotic performance (i.e., ultimate command of the language), they chose subjects who had at least 40 years of experience with the language as their primary, everyday communica- tion system. The subjects were tested on their production and compre- hension of ASL verb morphology. The results show a linear decline in

  • CRITICAL PERIOD 63

    performance with increasing age of exposure, on virtually every mor- pheme tested. That is, native learners scored better than early learners, who scored better than late learners, on both production and comprehen- sion.

    This study thus provides direct evidence that there is a decline over age in the ability to acquire a first language. It also tells us, however, that Lennebergs portrayal is at least partially incorrect in two regards. First, the results show a continuous linear decline in ability, instead of a sudden drop-off at puberty as his hypothesis implies. (This study does not tell us whether th

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